Electricity meters are utilized to measure consumption of energy by a load and include components such as current and voltage coils which are sensitive to external magnetic fields. For example, in an electronic meter, a processor (e.g., an application specific integrated circuit) receives input current and voltage signals from current and voltage coils, and uses such signals to generate a measure of energy consumption. The current and voltage coils are coupled to the power lines that supply energy to the load. If an external magnetic field interacts with the coils, meter accurancy can be adversely affected.
In an electromechanical meter, an aluminum disk is supported on a shaft and is driven to rotate by magnetic fields at a speed proportional to electric power being consumed by the load. The shaft is supported by a bearing so that very little friction acts on the shaft. A voltage coil and a current coil, positioned on respective, opposite sides of the disk, are coupled to the power lines supplying energy to the load. A permanent magnet, sometimes referred to as a retarding magnet, is spaced from the voltage and current coils and is positioned so that its poles are on opposite sides of the disk. The energized voltage and current coils generate magnetic fields which impart a rotational torque to the disk while the field of the permanent magnet imparts a retarding torque to the disk. If an external magnetic field interacts with the coils, the bearing, or the retarding magnet, accuracy of the meter can be adversely affected.
Since the readings of an electricity meter form the basis for income to electric utilities, it is important that the meter accurately measure energy consumption and not be subject to inaccuracies that might be due to tampering. For example, accuracy of an electronic meter can be adversely affected by a strong permanent magnet positioned near enough to the current coils to affect signals generated by the coils. Similarly, accuracy of an electromechanical meter can be adversely affected by a strong permanent magnet positioned near the coils, the bearing, or the retarding magnet.
Until recently, compact permanent magnets with sufficient magnetic strength to adversely affect meter components had not been generally available. Since such magnets are now generally available, it would be desirable to design electricity meters in a way that protects meter components from tampering by use of such magnets.
Recently, both electronic and electromechanical meter frames have been molded from plastic. In an electronic meter, the meter frame is configured to support the voltage and current coils, and the circuit boards are utilized to process signals from the coils. In an electromechanical meter, the meter frame supports the voltage and current coils, as well as the bearing and the retarding magnets.
Plastic molded meter frames are lower in cost than previously used die cast meter frames, facilitating more cost efficient energy consumption metering. However, these lower cost plastic molded frames can be even more susceptible to tampering with a permanent magnet, as described above, than metallic frames. Therefore, it would be desirable to provide an effective way of substantially reducing the opportunity for tampering without significantly increasing the cost of the plastic molded frame, so that the cost advantages of the plastic molded frame are substantially preserved. In addition, the tamper resistant feature of such frame should preferably be relatively indiscernible so as to avoid drawing attention to the magnetic shielding structure of the frame.